In oil well cementing, cement must flow through the casing before reaching the target annulus hence, a retarder must be added to provide the cement with sufficient time to reach the target depth before the cement sets. At the same time, in support of the Paris Agreement, the prospect of substituting ordinary Portland cement (OPC) with geopolymer cement as the well cement material has to be further explored. Although previous studies have found that retarders can delay the strength development of the cement, the studies were conducted either at ambient conditions or using OPC hence, the findings do not apply to geopolymer cement that is exposed to wellbore conditions. In order to address the shortcomings of the studies, the addition of a hydrocarboxylic‑acid-based retarder to a fly-ash-based geopolymer cement, at concentrations of up to 3% by weight of fly ash, was performed. Slurry of the cement was cured at 100 °C and 20.7 MPa for 8, 24 and 48 hours. Compressive strength tests were conducted on samples of the cement. At the 8-hour curing duration, retarder concentrations of 0.5–2.0% led to strength increases of 112.7–129.4% relative to that of 0%, or the control sample, whereas that of 3.0% led to a strength decrease of 84.2%. At the 24-hour curing duration, all retarder concentrations led to strength decreases of 16.4–22.5%. At the 48-hour curing duration, retarder concentrations of 1.0–3.0% led strength increases of 18.1–24.4%, whereas that of 0.5% led to a strength decrease of 16.7 %.